Rotary internal combustion engine with pilot subchamber

ABSTRACT

A rotary engine including a rotor sealingly received within an internal cavity of an outer body to define a plurality of combustion chambers having a variable volume, a pilot subchamber located in a wall of the outer body, the pilot subchamber in fluid communication with the internal cavity via at least two spaced apart transfer holes defining a flow restriction between the pilot subchamber and the internal cavity, a pilot fuel injector in fluid communication with the pilot subchamber, an ignition element configured for igniting fuel in the pilot subchamber, and a main fuel injector extending through the stator body and communicating with the cavity at a location spaced apart from the pilot subchamber. A method of combusting fuel in a rotary engine is also discussed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. application Ser. No. 14/706,457 filed May 7, 2015, which is a continuation of U.S. application Ser. No. 13/273,534 filed Oct. 14, 2011, which claims priority from provisional U.S. application No. 61/512,593 filed Jul. 28, 2011, the entire contents of all of which are incorporated by reference herein.

TECHNICAL FIELD

This application relates generally to rotary internal combustion engines, more particularly to the combustion in such engines.

BACKGROUND OF THE ART

Rotary engines, such as for example Wankel engines, use the eccentric rotation of a piston to convert pressure into a rotating motion, instead of using reciprocating pistons. In these engines, the rotor includes a number of apex or seal portions which remain in contact with a peripheral wall of the rotor cavity of the engine throughout the rotational motion of the rotor to create a plurality of rotating chambers when the rotor rotates.

Wankel engines are typically used with gasoline or similar fuel, with a single fuel injector or with two spaced apart fuel injectors. The fuel injector(s) may be located in a recess adjacent the combustion chamber and defined integrally through the engine housing, to communicate with an ignition member such as for example a spark plug. However, known arrangements are not optimized for use in a compound cycle engine system and/or for use with so-called heavy fuels, such as kerosene, and thus room for improvement exists.

SUMMARY

In one aspect, there is provided a rotary engine comprising a stator body having an internal cavity defined by two axially spaced apart end walls and a peripheral wall extending between the end walls, the cavity having an epitrochoid shape defining two lobes, a rotor body having three circumferentially spaced apex portions, the rotor body being engaged to an eccentric portion of a shaft to rotate and perform orbital revolutions within the cavity with each of the apex portions remaining in sealing engagement with the peripheral wall and separating three rotating chambers of variable volume defined in the cavity around the rotor body, an insert in the peripheral wall of the stator body, the insert being made of a material having a greater heat resistance than that of the peripheral wall, the insert having a subchamber defined therein and having an inner surface bordering the cavity, the subchamber communicating with the cavity through at least one opening defined in the inner surface and having a shape forming a reduced cross-section adjacent the opening, a pilot fuel injector having a tip received in the subchamber, an ignition element having a tip received in the subchamber, and a main fuel injector extending through the stator body and having a tip communicating with the cavity at a location spaced apart from the insert.

In another aspect, there is provided a stator body for a Wankel engine comprising two axially spaced apart end walls, a peripheral wall extending between the end walls and defining an internal cavity therewith, the cavity having an epitrochoid shape defining two lobes, an insert in the peripheral wall of the stator body, the insert being made of a material having a greater heat resistance than that of the peripheral wall, the insert having a subchamber defined therein and having an inner surface bordering the cavity, the subchamber communicating with the cavity through at least one opening defined in the inner surface and having a shape forming a reduced cross-section adjacent the opening, at least one of the insert and the peripheral wall having a pilot fuel injector elongated hole defined therethrough communicating with the subchamber and sized to receive a pilot fuel injector therein, at least one of the insert and the peripheral wall having an ignition element elongated hole defined therethrough communicating with the subchamber and sized to receive an ignition element therein, and the peripheral wall having a main fuel injector elongated hole defined therethrough spaced apart from the insert and sized to receive a main fuel injector therein.

In yet another aspect, there is provided a method of injecting heavy fuel into a Wankel engine having rotating chambers each having a volume varying between a minimum volume and a maximum volume, the method comprising injecting a minor portion of the heavy fuel into a subchamber defined adjacent to and in sequential communication with each of the rotating chambers and having a subchamber volume corresponding to from 5% to 25% of a sum of the minimum volume and the subchamber volume, igniting the heavy fuel within the subchamber, partially restricting a flow of the ignited heavy fuel from the subchamber to the rotating chambers, and injecting a remainder of the heavy fuel into each of the rotating chambers sequentially, independently of and spaced apart from the subchamber.

In another aspect, there is provided a rotary engine comprising: a rotor body mounted for eccentric revolutions within a stator body to provide rotating chambers of variable volume in an internal cavity of the stator body, the volume of each chamber varying between a minimum volume and a maximum volume; an insert in a peripheral wall of the stator body, the insert being made of a material having a greater heat resistance than that of the peripheral wall, the insert having a subchamber defined therein and having an inner surface, the subchamber communicating with the cavity through at least one opening defined in the inner surface and having a shape forming a reduced cross-section adjacent the opening, the subchamber having a volume corresponding to from 5% to 25% of a sum of the minimum volume and the volume of the subchamber; a pilot fuel injector having a tip received in the subchamber, the tip of the pilot fuel injector extending through an injector opening defined through the insert; an ignition element received within an ignition element hole defined through the insert, the ignition element having a tip received in the subchamber; and a main fuel injector extending through the stator body and having a tip communicating with the cavity at a location spaced apart from the insert.

In another aspect, there is provided a method of injecting heavy fuel into a Wankel engine having rotating chambers each having a volume varying between a minimum volume and a maximum volume, the method comprising: injecting a minor portion of the heavy fuel into a subchamber defined adjacent to and in sequential communication with each of the rotating chambers, the subchamber having a subchamber volume corresponding to from 5% to 25% of a sum of the minimum volume and the subchamber volume; igniting the heavy fuel within the subchamber; creating a hot wall around the subchamber by providing the subchamber in an insert received in a wall of a stator of the engine, the insert being made of a material more resistant to high temperature than that of the wall; partially restricting a flow of the ignited heavy fuel from the subchamber to the rotating chambers; and injecting a remainder of the heavy fuel into each of the rotating chambers sequentially, independently of and spaced apart from the subchamber.

In another aspect, there is provided a rotary engine comprising: a rotor sealingly received within an internal cavity of an outer body to define a plurality of combustion chambers; a pilot subchamber located in a wall of the outer body, the pilot subchamber in fluid communication with the internal cavity via at least two spaced apart transfer holes defining a flow restriction between the pilot subchamber and the internal cavity, the pilot subchamber in use in fluid communication with the combustion chambers as the rotor rotates; a pilot fuel injector in fluid communication with the pilot subchamber; an ignition element configured for igniting fuel in the pilot subchamber; and a main fuel injector communicating with the internal cavity independently from the pilot subchamber.

In another aspect, there is provided a compound engine assembly including the rotary engine as defined above, a compressor in fluid communication with the intake port of the rotary engine, and a turbine in fluid communication with the exhaust port of the rotary engine, the turbine having a turbine shaft compounded with an engine shaft drivingly engaged to the rotor.

In a further aspect, there is provided a method of combusting fuel in a rotary engine, the method comprising: injecting a minor portion of the fuel into a pilot subchamber; injecting a major portion of the fuel into a combustion chamber of the rotary engine independently of the pilot subchamber; igniting the fuel within the pilot subchamber; and igniting the fuel in the combustion chamber by directing the ignited fuel from the pilot subchamber into the combustion chamber in at least two discrete flows.

DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures in which:

FIG. 1 is a partial, schematic cross-sectional view of a rotary internal combustion engine in accordance with a particular embodiment;

FIG. 2 is a schematic cross-sectional view of an insert of the engine of FIG. 1;

FIG. 3 is a schematic cross-sectional view of an insert in accordance with another embodiment;

FIG. 4 is a schematic cross-sectional view of an insert in accordance with a further embodiment;

FIG. 5 is a schematic cross-sectional view of an engine assembly in accordance with a particular embodiment, in which the rotary internal combustion engine of FIG. 1 can be used;

FIG. 6 is a schematic view of part of an inner surface of a wall of the rotary engine of FIG. 1 in accordance with a particular embodiment; and

FIG. 7 is a schematic view of an end wall of a rotary engine of the rotary engine of FIG. 1 in accordance with another particular embodiment.

DETAILED DESCRIPTION

Referring to FIG. 1, a rotary internal combustion engine 10 known as a Wankel engine is schematically and partially shown. In a particular embodiment, the rotary engine 10 is used in a compound cycle engine system or compound cycle engine such as described in Lents et al.'s U.S. Pat. No. 7,753,036 issued Jul. 13, 2010, or such as described in Julien et al.'s U.S. Pat. No. 7,775,044 issued Aug. 17, 2010, or such as described in Thomassin et al.'s U.S. patent publication No. 2015/0275749 published Oct. 1, 2015, or such as described in Bolduc et al.'s U.S. patent publication No. 2015/0275756 published Oct. 1, 2015, the entire contents of all of which are incorporated by reference herein. The compound cycle engine system may be used as a prime mover engine, such as on an aircraft or other vehicle, or in any other suitable application. In any event, in such a system, air is compressed by a compressor before entering the Wankel engine, and the engine drives one or more turbine(s) of the compound engine. In another embodiment, the rotary engine 10 is used without a turbocharger, with air at atmospheric pressure. It is understood that the compound cycle engine system may have any other suitable configuration, including, but not limited to, a single shaft engine assembly.

FIG. 5 illustrates a compound engine assembly 10 in accordance with a particular embodiment, which may be configured for example as a turboprop engine, a turboshaft engine, a turbofan engine, or an auxiliary power unit (APU). The engine assembly 100 generally includes a compressor 120, an intermittent internal combustion engine 112 configured for example as a liquid cooled heavy fueled multi-rotor rotary intermittent combustion engine, and a turbine section 118 including one or more turbines.

The outlet of the compressor 120 is in fluid communication with the inlet of the engine 112; although not shown, such communication may be performed through an intercooler so as to reduce the temperature of the compressed air prior to the compressed air entering the engine 112. In the embodiment shown, the compressor 120 includes variable inlet guide vanes 123 through which the air flows before reaching the rotor(s) of the compressor 120. The compressor 120 may be a single-stage device or a multiple-stage device and may include one or more rotors having radial, axial or mixed flow blades.

A source of fuel 114 is in fluid communication with fuel injectors 42, 78 (further described below) of the engine 112. In a particular embodiment, the source of fuel 114 is a source of heavy fuel e.g. diesel, kerosene, jet fuel, equivalent biofuel; other suitable types of fuel may alternately be used, including, but not limited to, “light fuel” such as gasoline and naphtha. In the engine 112 the compressed air is mixed with the fuel and combusted to provide power and a residual quantity of exhaust gas. The engine 112 drives an engine shaft 116, and provides an exhaust flow in the form of exhaust pulses of high pressure hot gas exiting at high peak velocity. The outlet of the engine 112 is in fluid communication with the inlet of the turbine section 118, and accordingly the exhaust flow from the engine 112 is supplied to the turbine(s) of the turbine section 118.

The turbine section 118 includes at least one turbine rotor engaged on a turbine shaft 119. In a particular embodiment, the turbine section 118 includes a first stage turbine 122 receiving the exhaust from the engine 112, and a second stage turbine 124 receiving the exhaust from the first stage turbine 122; each turbine 122, 124 may be a single-stage device or a multiple-stage device and may include one or more rotors having radial, axial or mixed flow blades. In a particular embodiment, the turbines 122, 124 have different reaction ratios from one another. In a particular embodiment, the first stage turbine 122 is configured to take benefit of the kinetic energy of the pulsating flow exiting the engine 112 while stabilizing the flow and the second stage turbine 124 is configured to extract energy from the remaining pressure in the flow. Accordingly, in a particular embodiment the reaction ratio of the first stage turbine 122 is lower than the reaction ratio of the second stage turbine 124. Other configurations are also possible.

Power from the engine 112 and turbines 122, 124 is compounded to drive a rotatable load 108, for example via a gearbox 126 defining a driving engagement between the engine shaft 116, the turbine shaft 119 and the rotatable load 108. The rotatable load 108 may be any suitable type of load including, but not limited to, one or more generator(s), propeller(s), helicopter rotor mast(s), fan(s), compressor(s), or any other appropriate type of load or combination thereof. It is understood that the power from the engine shaft 116 and turbine shaft 119 may be compounded using any other suitable type of engagement, including, but not limited to, by having each shaft engaged to a respective electrical motor/generator with power being transferable between the electrical motor/generators (electrical compounding).

In the embodiment shown, the compressor 120 is driven by the turbine section 118, by having the rotor(s) of the compressor 120 directly engaged to the turbine shaft 119. Alternately, the rotor(s) of the compressor 120 may be connected to a separate shaft driven by the turbine shaft 119 and/or the engine shaft 116, for example via the gearbox 126 or via a separate gearbox.

It is understood that the engine assembly 100 may have other configurations than that of a compound engine assembly. For example, the turbine section 118 may be omitted, or may rotate independently of the internal combustion engine 112. The compressor 120 may be omitted. For example, the internal combustion engine 112 may have its inlet and outlet in direct communication with ambient air, i.e. be used without being fluidly connected to a compressor and a turbine.

In the embodiment shown, the engine 112 is a rotary intermittent internal combustion engine including two or more rotor assemblies 111 drivingly engaged to the engine shaft 116. In another embodiment, the engine 112 includes a single rotor assembly 111. In a particular embodiment, the rotor assembly(ies) 111 are configured as Wankel engines.

Referring back to FIG. 1, an example of a Wankel engine 10 which may define a rotor assembly 111 of the engine 112 is shown. It is understood that the configuration of the rotor assembly 111, e.g. placement of ports, number and placement of seals, number of apex portions, combustion chambers, etc., may vary from that of the embodiment shown.

The engine 10 comprises an outer body 12 having axially-spaced end walls 14 with a peripheral wall 18 extending therebetween to form a rotor cavity 20. The inner surface 19 of the peripheral wall 18 of the cavity 20 has a profile defining two lobes, which is preferably an epitrochoid.

An inner body or rotor 24 is received within the cavity 20, with the geometrical axis of the rotor 24 being offset from and parallel to the axis of the outer body 12. The rotor 24 has axially spaced end faces 26 adjacent to the outer body end walls 14, and a peripheral face 28 extending therebetween. The peripheral face 28 defines three circumferentially-spaced apex portions 30 (only one of which is shown), and a generally triangular profile with outwardly arched sides. The apex portions 30 are in sealing engagement with the inner surface of peripheral wall 18 to form three rotating combustion chambers 32 (only two of which are partially shown) between the inner rotor 24 and outer body 12. A recess 38 is defined in the peripheral face 28 of the rotor 24 between each pair of adjacent apex portions 30, to form part of the corresponding chamber 32.

The combustion chambers 32 are sealed. Each rotor apex portion 30 has an apex seal 52 extending from one end face 26 to the other and protruding radially from the peripheral face 28. Each apex seal 52 is biased radially outwardly against the peripheral wall 18 through a respective spring. An end seal 54 engages each end of each apex seal 52, and is biased against the respective end wall 14 through a suitable spring. Each end face 26 of the rotor 24 has at least one arc-shaped face seal 60 running from each apex portion 30 to each adjacent apex portion 30, adjacent to but inwardly of the rotor periphery throughout its length. A spring urges each face seal 60 axially outwardly so that the face seal 60 projects axially away from the adjacent rotor end face 26 into sealing engagement with the adjacent end wall 14 of the cavity. Each face seal 60 is in sealing engagement with the end seal 54 adjacent each end thereof.

Although not shown in the Figures, the rotor 24 is journaled on an eccentric portion of a shaft and includes a phasing gear co-axial with the rotor axis, which is meshed with a fixed stator phasing gear secured to the outer body co-axially with the shaft. The shaft rotates the rotor 24 and the meshed gears guide the rotor 24 to perform orbital revolutions within the rotor cavity. The shaft rotates three times for each complete rotation of the rotor 24 as it moves around the rotor cavity 20. Oil seals are provided around the phasing gear to prevent leakage flow of lubricating oil radially outwardly thereof between the respective rotor end face 26 and outer body end wall 14.

At least one inlet port (not shown) is defined through one of the end walls 14 or the peripheral wall 18 for admitting air (atmospheric or compressed) into one of the combustion chambers 32, and at least one exhaust port (not shown) is defined through one of the end walls 14 or the peripheral wall 18 for discharge of the exhaust gases from the combustion chambers 32. The inlet and exhaust ports are positioned relative to each other and relative to the ignition member and fuel injectors (further described below) such that during each rotation of the rotor 24, each chamber 32 moves around the cavity 20 with a variable volume to undergo the four phases of intake, compression, expansion and exhaust, these phases being similar to the strokes in a reciprocating-type internal combustion engine having a four-stroke cycle.

In a particular embodiment, these ports are arranged such that the rotary engine 10 operates under the principle of the Miller or Atkinson cycle, with its volumetric compression ratio lower than its volumetric expansion ratio. In another embodiment, the ports are arranged such that the volumetric compression and expansion ratios are equal or similar to one another.

An insert 34 is received in a corresponding hole 36 defined through the peripheral wall 18 of the outer body 12, for pilot fuel injection and ignition. The peripheral wall 18 also has a main injector elongated hole 40 defined therethrough, in communication with the rotor cavity 20 and spaced apart from the insert 34. A main fuel injector 42 is received and retained within this corresponding hole 40, with the tip 44 of the main injector 42 communicating with the cavity 20 at a point spaced apart from the insert 34. The main injector 42 is located rearwardly of the insert 34 with respect to the direction R of the rotor rotation and revolution, and is angled to direct fuel forwardly into each of the rotating chambers 32 sequentially with a tip hole pattern designed for an adequate spray.

Referring particularly to FIG. 2, the insert includes an elongated body 46 extending across a thickness of the peripheral wall 18, with an enlarged flange 48 at its outer end which is biased away from a shoulder 50 defined in the peripheral wall 18, and against a gasket (not shown) made of an appropriate type of heat resistant material such as a silica based material. A washer 56, such as for example a steel or titanium washer, and spring 58, such as for example a wave spring or a Belleville spring, are provided between the flange 48 and the shoulder 50 of the peripheral wall 18. The spring 58 biases the body 46 against a cover 62 having a cross-section greater than that of the hole 36 and extending over an outer surface 64 of the peripheral wall 18. The cover 62 is connected to the peripheral wall 18, for example through brazing. Alternate types of connections can also be used, including but not limited to a connection through fasteners such as bolts, to help facilitate replacement of the insert if necessary.

The insert body 46 has an inner surface 66 which is continuous with the inner surface 19 of the peripheral wall 20 to define the cavity 20. The insert hole 36 in the wall 18 defines a flange 68 extending in the insert hole 36 adjacent the inner surface 19, and the inner end of the insert body 46 is complementarily shaped to engage this flange 68, with a gasket 70 being received therebetween.

The insert body 46 is made of a material having a greater heat resistance than that of the peripheral wall 18, which in a particular embodiment is made of aluminium. In this particular embodiment, the insert body 46 is made of an appropriate type of ceramic.

The insert body 46 has a pilot subchamber 72 defined therein in communication with the rotor cavity 20. In the embodiment shown, the subchamber 72 has a circular cross-section; alternate shapes are also possible. The subchamber 72 communicates with the cavity through at least one opening 74 defined in the inner surface 66. The subchamber 72 has a shape forming a reduced cross-section adjacent the opening 74, such that the opening 74 defines a restriction to the flow between the subchamber 72 and the cavity 20. The opening 74 may have various shapes and/or be defined by a pattern of multiple transfer holes.

Referring to FIG. 6, an example of communication between the subchamber 72 and the internal cavity including multiple spaced apart transfer holes 75 is shown. In the embodiment shown, four separate transfer holes 75 are provided; it is understood that more or less transfer holes 75 may alternately be provided. In the embodiment shown, the transfer holes 75 are located in a diamond pattern; it is understood that any other suitable pattern may alternately be used.

Referring back to FIG. 2, the peripheral wall 18 has a pilot injector elongated hole 76 defined therethrough in proximity of the insert 34, extending at a non-zero angle with respect to a surface of an outer wall of the insert 34, and in communication with the subchamber 72. A pilot fuel injector 78 is received and retained within the corresponding hole 76. The pilot fuel injector 78 is in fluid communication with the subchamber 72, for example by having the tip 80 of the pilot injector 78 received in the subchamber 72. As can be seen in FIG. 2, the insert body 46 has an injector opening defined therethrough providing the communication between the pilot injector elongated hole 76 and the subchamber 72, and the tip 80 of the pilot injector 78 is received in the subchamber 72 through this injector opening, with a major part of the pilot injector 78 being received in the pilot injector elongated hole 76 outside of the insert 34.

The insert body 46 and cover 62 have an ignition element elongated hole 82 defined therein extending along the direction of the transverse axis T of the outer body 12, also in communication with the subchamber 72. An ignition element 84 is received and retained within the corresponding hole 82 and is configured for fuel ignition in the subchamber 72, for example by having the tip 86 of the ignition element 84 received in the subchamber 72. In the embodiment shown, the ignition element 84 is a glow plug in heat exchange relationship with the subchamber 72. Alternate types of ignition elements 84 which may be used include, but are not limited to, plasma ignition, laser ignition, spark plug, microwave, etc.

The pilot injector 78 and main injector 42 inject fuel, and in a particular embodiment heavy fuel (e.g. diesel, kerosene, jet fuel, equivalent biofuel) into the chambers 32; the pilot injector 78 injects a minor portion of the fuel while the main injector 42 injects a major portion of the fuel. In a particular embodiment, at least 0.5% and up to 20% of the fuel is injected through the pilot injector 78, and the remainder is injected through the main injector 42. In another particular embodiment, at most 10% of the fuel is injected through the pilot injector 78. In another particular embodiment, at most 5% of the fuel is injected through the pilot injector 78. The main injector 42 injects the fuel such that each rotating chamber 32 when in the combustion phase contains a lean mixture of air and fuel.

Referring to FIG. 3, an insert 134 according to another embodiment is shown, engaged to the same outer body 12. The insert 134 extends across a thickness of the peripheral wall 18, and includes an inner body portion 146 and an outer body portion 162 which are attached together, for example through a high temperature braze joint 188. The outer body portion 162 has an enlarged flange 148 at its outer end which abuts the outer surface 64 of the peripheral wall 18 and is connected thereto, for example through bolts with appropriate sealing such as a gasket or crush seal (not shown). Alternate types of connections can also be used, including but not limited to a brazed connection.

The inner body portion 146 has an inner surface 166 which is continuous with the inner surface 19 of the peripheral wall 18 to define the cavity 20. The inner end of the inner body portion 146 is complementarily shaped to engage the flange 68 extending in the insert hole 36 adjacent the inner surface 19, with a gasket 70 being received therebetween.

In this particular embodiment, the body portions 146, 162 are made of an appropriate type of super alloy such as a Nickel based super alloy.

The pilot subchamber 72 is defined in the insert 134 at the junction between the body portions 146, 162, with the inner body portion 146 defining the opening 74 for communication between the subchamber 72 and the cavity 20. The outer body portion 162 has the ignition element elongated hole 82 defined therein along the direction of the transverse axis T and in communication with the subchamber 72. The ignition element 84 is received and retained within the corresponding hole 82, for example through threaded engagement. As in the previous embodiment, the tip 86 of the ignition element 84 is received in the subchamber 72.

Referring to FIG. 4, an insert 234 according to another embodiment is shown. The insert 234 is received in a corresponding hole 236 defined through the peripheral wall 18. The insert 234 includes an inner body portion 246 and an outer body portion 262 which are attached together, for example through a high temperature braze joint, with the subchamber 72 being defined at the junction of the two portions 246, 262. The inner body portion 246 defines the opening 74 for communication between the subchamber 72 and the cavity 20.

The outer body portion 262 has the ignition element elongated hole 82 defined therethrough in communication with the subchamber 72. The outer body portion 262 includes an inner enlarged section 245 connected to the inner body portion 246 and defining the subchamber 72. The enlarged section 245 extends substantially across the width of the hole 236 around the subchamber 72, then tapers to a reduced width section 247 extending therefrom. The reduced width section 247 has at its outer end an enlarged flange 248 which abuts a shoulder 250 defined in the outer surface 64 of the peripheral wall 18 around the hole 236. An outer section 249, which in the embodiment shown has a width intermediate that of the sections 245 and 247, extends outwardly from the flange 248. The flange is connected to the shoulder, for example through bolts (not shown) with appropriate sealing such as a crush seal or a gasket (not shown) made of high temperature material, for example a silica based material or grafoil, between the flange 248 and shoulder 250. Alternate types of connections can also be used.

The inner body portion 246 has an inner surface 266 which is continuous with the inner surface 19 of the peripheral wall 18 to define the cavity 20. The inner body portion 246 includes a groove defined therearound near the inner surface 266, in which an appropriate seal 251, for example a silica based gasket tape, is received in contact with the walls of the insert hole 236. In this embodiment, the walls of the insert holes 236 are straight adjacent the inner surface 19, i.e. there is no flange adjacent the inner surface 19.

The volume of the subchamber 72 in the insert 34, 134, 234 is selected to obtain a stoichiometric mixture around ignition within an acceptable delay, with some of the exhaust product from the previous combustion cycle remaining in the subchamber 72. In a particular embodiment, the volume of the subchamber 72 is at least 0.5% and up to 3.5% of the displacement volume, with the displacement volume being defined as the difference between the maximum and minimum volumes of one chamber 32. In another particular embodiment, the volume of the subchamber 72 corresponds to from about 0.625% to about 1.25% of the displacement volume.

The volume of the subchamber 72 may also be defined as a portion of the combustion volume, which is the sum of the minimum chamber volume Vmin (including the recess 38) and the volume of the subchamber V₂ itself. In a particular embodiment the subchamber 72 has a volume corresponding to from 5% to 25% of the combustion volume, i.e. V₂=5% to 25% of (V₂+V_(min)). In another particular embodiment, the subchamber 72 has a volume corresponding to from 10% to 12% of the combustion volume, i.e. V₂=10% to 12% of (V₂+V_(min)).

The subchamber 72 may help create a stable and powerful ignition zone to ignite the overall lean main combustion chamber 32 to create the stratified charge combustion. The subchamber 72 may improve combustion stability, particularly but not exclusively for a rotary engine which operates with heavy fuel below the self-ignition of fuel. The insert 34, 134, 234 made of a heat resistant material may advantageously create a hot wall around the subchamber which may further help with ignition stability.

It is understood that the subchamber 72 may alternately be defined in or adjacent one of the end walls 14, as illustrated by FIG. 7. In this embodiment, the fluid communication between the subchamber 72 and the internal cavity is defined via three separate transfer holes extending through the end wall 14. It is understood that more or less transfer holes 75 may alternately be provided. In the embodiment shown, the transfer holes 75 are located in a linear pattern; it is understood that any other suitable pattern may alternately be used.

It is also understood that the subchamber 72 may be defined directly in the peripheral wall 18 or end wall 14, i.e. without being defined in an insert made of a different material, or may be defined in any other suitable type and configuration of insert. For example, the embodiments of FIGS. 6-7 include, but are not intended to be limited to, the insert configurations shown in FIGS. 1-4.

In a particular embodiment and in use, the fuel is combusted in accordance with the following. A minor portion of the fuel is injected into the pilot subchamber 72, and a major portion of the fuel is injected into a combustion chamber 32 independently of the pilot subchamber 72. The fuel within the pilot subchamber 72 is ignited, for example with the ignition element 84. The fuel in the combustion chamber 32 by directing the ignited fuel from the pilot subchamber 72 into the combustion chamber 32. In a particular embodiment, this is performed in at least two discrete flows, via the separate transfer holes 75 (FIGS. 6-7) each defining one of the flows. Other configurations are also possible.

The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention(s) disclosed. For example, the mechanical arrangement of the Wankel engine described above is merely one example of many possible configurations which are suitable for use with the present invention(s). Any suitable injector configuration and arrangement may be used. Hence, modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims. 

1. A rotary engine comprising: a rotor sealingly received within an internal cavity of an outer body to define a plurality of combustion chambers; a pilot subchamber located in a wall of the outer body, the pilot subchamber in fluid communication with the internal cavity via at least two spaced apart transfer holes defining a flow restriction between the pilot subchamber and the internal cavity, the pilot subchamber in use in fluid communication with the combustion chambers as the rotor rotates; a pilot fuel injector in fluid communication with the pilot subchamber; an ignition element configured for igniting fuel in the pilot subchamber; and a main fuel injector communicating with the internal cavity independently from the pilot subchamber.
 2. The engine as defined in claim 1, wherein an insert is received in a peripheral wall of the stator body, the insert being made of a material having a greater heat resistance than that of the peripheral wall, the insert having the pilot subchamber defined therein, the at least two transfer holes extending through the insert.
 3. The engine as defined in claim 2, wherein the insert is made of ceramic or super alloy.
 4. The engine as defined in claim 2, wherein the ignition element is received within an ignition element hole defined through the insert.
 5. The engine as defined in claim 2, wherein the pilot injector extends through the peripheral wall at a non-zero angle with respect to a surface of an outer wall of the insert with only a portion thereof extending within the insert.
 6. The engine as defined in claim 2, wherein the pilot injector extends through the peripheral wall at a non-zero angle with respect to a longitudinal direction of the insert.
 7. The engine as defined in claim 1, wherein the pilot subchamber has a volume corresponding to from 5% to 25% of a sum of a minimum volume of one of the combustion chambers and the volume of the pilot subchamber.
 8. The engine as defined in claim 7, wherein the volume of the pilot subchamber corresponds to from 10% to 12% of the sum of the minimum volume and the volume of the pilot subchamber.
 9. The engine as defined in claim 1, wherein a difference between a maximum volume and a minimum volume of one of the combustion chambers defines a displacement volume, the subchamber having a volume of at least about 0.5% of the displacement volume and at most about 3.5% of the displacement volume.
 10. The engine as defined in claim 1, wherein a difference between a maximum volume and a minimum volume of one of the combustion chambers defines a displacement volume, the pilot subchamber having a volume of at least 0.625% of the displacement volume.
 11. The engine as defined in claim 1, wherein a difference between a maximum volume and a minimum volume of one of the combustion chambers defines a displacement volume, the pilot subchamber having a volume of about 1.25% of the displacement volume.
 12. The engine as defined in claim 1, further comprising a heavy fuel source in communication with the main and pilot fuel injectors.
 13. The engine as defined in claim 1, wherein the ignition element is a glow plug.
 14. The rotary engine as defined in claim 1, wherein the rotary engine is a Wankel engine, the outer body including a peripheral wall cooperating with two spaced apart end walls to define the internal cavity, the peripheral wall defining two lobes, the rotor having three circumferentially-spaced apex portions in sealing engagement with the peripheral wall and separating the combustion chambers.
 15. A compound engine assembly including the rotary engine as defined in claim 1, a compressor in fluid communication with the intake port of the rotary engine, and a turbine in fluid communication with the exhaust port of the rotary engine, the turbine having a turbine shaft compounded with an engine shaft drivingly engaged to the rotor.
 16. A method of combusting fuel in a rotary engine, the method comprising: injecting a minor portion of the fuel into a pilot subchamber; injecting a major portion of the fuel into a combustion chamber of the rotary engine independently of the pilot subchamber; igniting the fuel within the pilot subchamber; and igniting the fuel in the combustion chamber by directing the ignited fuel from the pilot subchamber into the combustion chamber in at least two discrete flows.
 17. The method as defined in claim 16, wherein the pilot subchamber has a volume corresponding to from 5% to 25% of a sum of a minimum volume of the combustion chamber and the volume of the pilot subchamber volume.
 18. The method as defined in claim 17, wherein the volume of the pilot subchamber corresponds to from 10% to 12% of the sum of the minimum volume and the volume of the pilot subchamber.
 19. The method as defined in claim 16, further comprising creating a hot wall around the pilot subchamber by providing the pilot subchamber in an insert received in a wall of a stator of the engine, the insert being made of a material more resistant to high temperature than that of the wall.
 20. The method as defined in claim 19, wherein the insert is received in a peripheral wall of the engine.
 21. The method as defined in claim 19, wherein the insert is made of ceramic or super alloy.
 22. The method as defined in claim 16, wherein injecting the minor portion of the fuel into the pilot subchamber includes injecting from 0.5% to 20% of the fuel.
 23. The method as defined in claim 16, wherein injecting the minor portion of the fuel into the pilot subchamber includes injecting from 5% to 10% of the fuel.
 24. The method as defined in claim 16, wherein injecting the minor portion of the fuel is done in an angled direction with respect to a central transverse axis of a stator of the engine.
 25. The method as defined in claim 16, wherein the fuel is heavy fuel.
 26. The method as defined in claim 16, wherein the rotary engine is a Wankel engine and the combustion chamber is one of three combustion chambers, the engine having a housing including a peripheral wall cooperating with two spaced apart end walls to define an internal cavity, the peripheral wall defining two lobes, and having a rotor including three circumferentially-spaced apex portions in sealing engagement with the peripheral wall and separating the three combustion chambers. 